Core Data Migration: How to populate new attribute?

Question

I have what feels should be a simple problem, but cannot find a simple answer.

I have a simple migration, I just need to add a synthentic property for use in a Fetched Results Controller. This new property is just a BOOL (used for sorting) that is derived from another property.

Example:

var title: String? // "engineer" | "accountant"
var hasTitle: Bool // title != nil

Simply, I need to add the new field hasTitle (which seems trivial), and the populate it once. I want to keep this code out of my normal app logic, so I don't have to code in a history of all my schema changes.

Is it possible to do a lightweight migration followed by an isolated, one-time mass update, or do I have to do a custom / heavyweight migration.

And secondly, if I need to do a custom migration, are there any mitigating techniques - can I use a Value Expression "Function" to do initialize the above (title != nil)

What I want to avoid is having to set a NSUserDefaults flag for a migration, and check that on every launch. I would like to contain the complexity of the migration to migration specific code and not pollute the regular app logic.

Thanks!

Answer 1

Is it possible to do a lightweight migration followed by an isolated, one-time mass update, or do I have to do a custom / heavyweight migration.

Both are possible. I'd go with a custom migration, simply because it'll save you from having code that needs to check on the new attribute every time the app launches. For a simple change like yours it's pretty straightforward, although it's uncharted territory for most iOS developers. I recently described the process in detail in another answer.

If you don't go with a custom migration, you'd need to make the new attribute optional and then add some custom code to check if a value exists and assign one if it doesn't. It might seem simpler, but that's just because it probably seems more familiar. Core Data will save you from needing to insert this kind of check in your app launch process, and ensure that your conversion code runs only when a conversion is actually needed.